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Break down the costs of stainless-steel fabrication, and finishing would likely take an uncomfortably large portion.

According to Ernie Leopold, product manager at Pittsburgh-based Fein Power Tools, grinding and polishing can take up to 75 percent of final production costs. Even for shops purchasing pre-finished material, shipping and fabrication sometimes make manual grinding and polishing inevitable in the end. ("A good welder can be a grinder's best friend," Leopold points out.)

Regardless, if a shop doesn't first prevent contamination of the stainless material, even the best power tools won't help matters. With contamination under control, modern power tools can improve efficiency and help take a bite out of what remains one of the most labor-intensive aspects of sheet-metal work.

CONTROLLING CONTAMINATION

"A lot of fabricators who do stainless are not aware of the contamination factor and, hence, make some basic missteps," says Leopold.

Missteps include the use of conventional discs embedded with ferrous materials that, when exposed to stainless materials, can cause contamination resulting in corrosion or rust. The grit in discs applied to stainless must, he says, have aluminum oxide, zirconia or alternative material that won't cause contamination. Also, of course, never grind stainless with any disc previously used on carbon steel. "If it's been used on a ferrous product, never use it on a stainless product," he says.

Contamination can also come from the use of pneumatic tools, Leopold adds. Oils and moisture from the compressor can build up inside the ferrous piping and, ultimately, exit the exhaust ports of the pneumatic tool onto the stainless material.

Shops working in both carbon steel and stainless must take special care. "If you grind on carbon steel and shower nearby stainless material with sparks and debris, you've now contaminated the surface of that stainless." For this reason, he says, many high-end stainless shops don't bring any carbon steel onto the floor.

The level of finish in stainless can also affect contamination. A rough finish has more peaks and valleys on the surface to hold contaminated materials.

Grinding too much also causes problems with some alloys and geometries. Tubes ground with a conventional disc, leaving microscopic "flat spots" around the circumference, are especially susceptible to this. (Special tools for tube and pipe, covered later, help avoid this.) Alloy elements like nickel and chromium (common in the make-up of stainless steels) develop a passive, protective layer on the material surface. The more that layer is intact after finishing, the greater its "passive ability" to chemically re-generate this barrier.

Grinding removes the passive layer, so shops will leave finished stainless on the floor for several days to re-generate before exposing it to corrosive atmospheres. In general, grinding can significantly alter the metallurgical composition of stainless steel through the introduction of foreign materials and over-heating. In extreme cases, the passivisation process can be lost altogether, destroying the stainless steel's corrosion-resistant characteristics.

CONTROLLING TORQUE

Features in higher-end power tools have edged up process efficiency. Consider the constant-torque motor. In a typical cycle of a pneumatic power tool the operator presses the trigger, then moves the free-spinning wheel toward the workpiece. As the wheel hits, friction between the sheet and wheel causes the motor to lose substantial torque and power. For many grinding applications the loss doesn't cause concern. But for stainless and other material requiring highly polished surfaces, problems emerge. On the surface appear skid marks that must be polished out, which takes time. "If you are constantly losing your motor speed and torque, then you will have irregular finishes with your material," Leopold says.

Throughout a cycle, the grinding media will slow down and speed up, contributing to surface imperfections. Experienced operators feel these changes, consciously or subconsciously, and so adjust pressure to suit. But what if an operator hasn't developed this feel? Enter the constant-torque motor, which senses changes in power. "The motor reacts to the pressure of the wheel on the stock," Leopold explains. "It electronically adjusts the input to the motor and its ability to remain at a constant speed."

DIFFICULT GEOMETRIES

Unusual geometries create problems. Not only can small-radius corners, along with tube and pipe, put a damper on productivity, safety concerns also arise. It's not uncommon to see operators remove the guard on a disc grinder for hard-to-reach areas. "That's a big OSHA violation," Leopold says.

For maximum productivity and safety, operators need the proper tools, he explains, like miniature disc grinders, die grinders and small belt grinders. "For corners, inside radii and areas like inside pressure vessels, you just can't get a standard grinder to do the job."

Finishing stainless tube and pipe presents its own issues. Any disc grinder, regardless of operator skill, will produce small flat spots around the circumference. These spots, though minute, are potentially detrimental. "Under a magnifying lens, there would appear very narrow, flat areas around the tube or pipe," Leopold says. "You would no longer have a consistent circumference."

These spots happen due to the very small contact area between the tube and grinding-wheel edge; the smaller the contact area, the greater the pressure concentration—hence, the flat spots. "And if you work with extremely thin-gauge material, you run the risk of breaking through the metal," Leopold says.

For stainless, those flat spots also represent areas where the material's "passive" layers have been completely ground through; hence, the metal could easily become contaminated.